The orientation of an airplane in space is commonly described in terms of its roll, pitch and yaw. Roll refers to rotation about a roll axis that generally extends from the nose to the tail of the aircraft. Pitch is angular rotation about a pitch axis that is normal to the roll axis, and that generally passes through the wings of the aircraft. Yaw is angular rotation about a yaw axis that is normal to the pitch and roll axes, and that is perpendicular to the ground during level flight.
The lift-generating surfaces of an aircraft are highly asymmetric, and significant lift can, in general, be generated only along the yaw axis. Therefore, to turn an aircraft in level flight to the right or left, it is necessary to first roll the plane such that its yaw axis is no longer vertical, so that the lift vector generated by the wings has a component in the desired turn direction. This maneuver is generally described by the phrase "roll to turn".
One well-known type of missile comprises an approximately symmetric body, with fins projecting from the body near the tail end of the missile. In a so-called "cruciform" design, the fins are spaced 90.degree. from one another about the longitudinal, roll axis of the missile. Because of its symmetry, such a missile can, in principal, generate lift in any direction normal to the roll axis. However, it is usual that such missiles can generate more lift, and/or fly at higher ratios of lift to drag, when the lift vector is in some direction rather than in others.
For such missiles, it is advantageous to orient the missile so that the direction of the maximum lift capability is in the direction of the desired acceleration. This consideration applies to cruciform missiles of the type described above. In particular, a cruciform missile can accelerate normal to its flight path more rapidly in a plane that lies between the fins than in a plane containing a pair of the fins. This characteristic is summarized by stating that such missiles accelerate better in an "x" configuration (relative to the plane of the desired acceleration) than in a "+" configuration. It is therefore preferable, in prior art missile control systems, to orient the autopilot pitch plane such that it lies between the fins. Then, to maneuver such a missile in a given lateral direction, the autopilot pitch plane is aligned with the desired movement direction by causing the missile to roll either to the right or left until the missile is in the desired x configuration relative to the plane containing the desired acceleration.
From the above, it may be seen that if the pitch autopilot is implemented such that it can maneuver equally well in both directions in the pitch plane, then it will generally be necessary for the missile to rotate up to 90.degree. clockwise or counterclockwise about the roll axis before or while accelerating laterally, in order to align the pitch plane with a desired command direction. If the pitch autopilot has a preferred direction in the pitch plane then rotation of up to 180.degree. may be required. The large roll angles that may be required are a significant limitation of the missile responsiveness to commanded changes in its flight path direction.
In order to obtain good, or in some cases, even adequate autopilot performance, the pitch and yaw autopilot channels must be different, even though the missile is symmetrical. The reason is that the airflow patterns around the missile are only symmetrical in the yaw (X,Y) plane when the missile is at a non-zero angle of attack and at zero sideslip. This causes the aerodynamic stability derivatives to be different in pitch and yaw, which, in turn, requires different gains in the pitch and yaw autopilot channels. This is the reason for requiring the autopilot pitch plane to be in the direction of the commanded maneuver. In the prior art, this necessitated rolling the missile as described above.